This invention relates to a method for carrying out well construction, repair and abandonment operations with a thermosetting resin as seal, plug or connection. More in particular, this invention relates to a method of improving the gas tightness of sealing materials in primary cementing, well repair and plugging operations in oil/gas wells.
This invention also relates to an analyzer for determining the setting time, elastic properties, shrinkage/expansion, compressibility and coefficient of thermal expansion of thermosetting resins and oil well cements under simulated reservoir pressure and temperature conditions.
The main objectives for drilling an oil or gas well are to create a connection to an oil and/or gas reservoir and to install a conduit (called production tubing) between the reservoir and the surface. The outer steel protection of a well is called the casing. The casing requires a gas tight seal between the reservoir and the surface. To achieve such seal, the annulus (the gap between the casing and the rock/formation) is subjected to a cementing (or grouting) operation. This treatment is normally referred to as Primary Cementing. The main aspects of primary cementing are to isolate flow between different reservoirs, to withstand the external and internal pressures acting upon the well by offering structural reinforcement and to prevent corrosion of the steel casing by chemically aggressive reservoir fluids.
A poor cementing job can result in migration of reservoir fluids, even leading to gas migration through micro-annuli in the well which not only reduces the cost-effectiveness of the well but may cause a xe2x80x9cblow outxe2x80x9d resulting in considerable damage. Repair jobs (xe2x80x9csecondary cementingxe2x80x9d) are possible (in essence forcing more cement into the cracks and micro-annuli). However, they are costly and do not always lead to the desired results.
When a well has reached the end of its economical life, the well needs to be abandoned in compliance with local regulations. Abandonment is usually carried out by first plugging each of the casings in a large number of sequential steps, cutting and removing these cut steel casing stubs and successively placing large cement plugs in order to permanently seal the well. As only relatively small volumes of cement (typically in the order of 100 m) are used for those plugs, their quality needs to be excellent as they will have to act as a seal for a very long time.
One of the major drawbacks of using traditional cementing materials such as xe2x80x98OPCxe2x80x99 Class G Oil Well Cement (xe2x80x98OPCxe2x80x99=Ordinary Portland Cement) in plugging operations is that this widely employed material cannot achieve a gas tight seal due to its inherent shrinkage. The total chemical contraction can be split between a bulk or external volume shrinkage (less than 1%), and a matrix internal contraction representing 4-6% by volume of the cement slurry, depending upon the cement composition (Parcevault, P. A. and Sault, P. H. xe2x80x98Cement Shrinkage and Elasticity: A new Approach for good zonal Isolationxe2x80x99, paper SPE 13176 (1984). The combined shrinkage phenomena cause gas migration through micro-annuli and cracks. These are created because of those shrinkage phenomena and the inherently poor adhesion of Oil Well cement to the steel Casing. The already poor adhesion is even further deteriorated by the inability to properly clean the surface of the steel casing prior to cementing.
The use of conventional cementing materials in xe2x80x9cremedial secondary cementingxe2x80x9d has the disadvantage that the customary grain size is too large to pass freely into the micro-annuli and cracks which affect the quality of the seal.
In the search for effective cementing materials, attention has to be paid to inter alia the following requirements: the material should be gas-tight (i.e. withstand at least 2 bar per m), it should have a controllable setting time so that a range of temperatures and well depths (each requiring different conditions) can be coped with, it should be thermally stable up to 250xc2x0 C. as well as being chemically stable against reservoir fluids for a very long period of time and its rheological properties should be such that pumping through existing oil field equipment can be carried out without too much problems.
A wide range of non cementious sealants have been suggested to cope with at least part of the problems outlined herein above. Examples of such materials are resins (R. Ng and C. H. Phelps: xe2x80x9cPhenolic/Epoxy Resins for water/Gas Profile Modification and Casing Leak Repairxe2x80x9d Paper ADSPE #90, presented at the ADIPEC, held in Abu Dhabi (16-19) October 1994); phenol-or melamine formaldehyde (W. V. C. de Landro and D. Attong: xe2x80x9cCase History: Water Shut-off using Plastic Resin in a High Rate Gravel pack Wellxe2x80x9dxe2x80x94Paper SPE 36125 presented at the 4th Latin American and Caribbean Petroleum Engineering Conference, held at Port of Spain in Trinidad, 23-26 Apr. 1996); and polyacrylates (U.S. Pat. No. 5,484,020 assigned to Shell Oil).
Although such materials can be instrumental in solving some of the problems encountered with traditional, cement-based plugs, there are still important drawbacks to be reckoned with in terms of handling aspects, control of setting times and long term durability.
Also rubbers have been proposed in general for use as plugging materials. Reference is made to U.S. Pat. No. 5,293,938 (assigned to Halliburton Company) directed to the use of compositions consisting essentially of a mixture of a slurry of a hydraulic cement (such as Portland cement) and a vulcanizable rubber latex. Rubbers specifically referred to in said U.S. patent specification are natural rubbers, cis-polyisoprene rubber, nitrile-rubber, ethylene-propylene rubber, styrene butadiene rubber, butyl rubber and neoprene rubber.
The vulcanization of the rubber involves the cross-linking of the polymer chains which can be accomplished by incorporating one or more cross-linking agents (the most common one being sulphur) in the rubber latex (latex having been defined as the aqueous dispersion or emulsion of the rubber concerned).
In European patent No. 325,541 (Merip Tools International S.A) the use of putty (xe2x80x9cmasticxe2x80x9d) has been disclosed for producing joints separating zones in wells. Suitable compounds are formed by liquid elastomers such as fluorosilicones, polysulphides, polythioethers as well as epoxy or phenolic resins. In addition, from International application WO 99/43923 a special class of room temperature vulcanizing silicone and fluorsilicone rubbers is known that can be advantageously employed in the repair and abandonment of wells.
Unfortunately, curing of non cementious plugging agents is also accompanied by shrinkage, again potentially leading to micro-annuli and cracks in the sealant and/or lack of bonding of the seal, plug or connection to its surroundings. This will especially occur if the adhesion of the thermosetting resin to the steel casing surface is less than the forces induced by the shrinkage process. It therefore remains desirable to further improve existing methods to overcome said drawbacks.
In accordance with the main embodiment of the invention there is provided a method for carrying out well construction, repair and abandonment operations, which method involves introducing a resin into a well and curing the same to form a seal, plug or connection, wherein the cured resin is expanded to at least the volume occupied by the resin prior to curing (compensating shrinkage), by cooling the well and curing the resin at a reduced temperature and subsequently allowing the well to reach its static bottom hole temperature.
The expression xe2x80x9cresinxe2x80x9d used in the main claim and throughout the specification refers to xe2x80x9cclassicxe2x80x9d thermosetting resins, as well as ductile, vulcanizable rubbers.
Another embodiment of the invention comprises a method for removing a seal, plug or connection made of an expanded resin and used in well construction, repair and abandonment, comprising the steps of a) cooling the well, until the seal, plug or connection has shrunk loose, and b) removing the loose seal, plug or connection.
Finally, the invention also provides a method for analyzing the setting time, elastic properties or shrinkage/expansion of resins or cements used in well construction, repair and abandonment operations under simulated reservoir pressure and temperature conditions, and the analyzer used by that method.